The present invention relates in general to communication systems, and is particularly directed to a new and improved echo cancellation mechanism for use within a digital communications transceiver coupled to a two-wire loop, and which serves to effectively mitigate against non-linear signal perturbations that may cause disruption of digital communication services, such as, but not limited to, those caused by micro-interruptions, in the two-wire loop providing high bit-rate digital subscriber line (HDSL) service, digital data service (DDS), integrated services digital network (ISDN) service, or HDSL2 service, that are transportable by transceivers over the two-wire loop.
In a typical telecommunication network for providing full-duplex data communications there is a first transceiver at a central office coupled via a two wire loop to a second transceiver at a second location. The first transceiver is usually referred to as an upstream transceiver and the second transceiver is referred to as a downstream transceiver. A signal transmitted from the first transceiver to the second transceiver travels in the downstream direction and conversely a signal transmitted towards the first transceiver travels in the upstream direction. Each of the transceivers has a transmitter and receiver. There are several methods to separate the upstream and downstream signals when the signals arrive at their respective receivers. The methods include time division multiplexing and frequency division multiplexing.
However when neither of the above multiplexing methods is used and when there is a significant frequency overlap of the downstream and upstream signals, echo canceling methods may be used. Echo cancellation methods have been in use since the early 1960s (e.g., attention may be directed to M. M. Sondhi, xe2x80x9cAn Adaptive Echo Canceller,xe2x80x9d Bell System Technical Journal, Vol. 48, No.3, March 1967, pp. 497-511, and F. K. Becker et al, xe2x80x9cApplication of Automatic Transversal Filters to the Problem of Echo Suppression,xe2x80x9d Bell Technical Journal, Vol. 45, No. 12, March 1966, pp. 1847+) and are also discussed in books such as xe2x80x9cThe Communications Handbook,xe2x80x9d edited by Jerry D. Gibson, CRC Press, 1997. Typically, echo cancelers are linear echo cancelers and in the present application linear echo cancelers are also referred to as conventional cancelers. Echos are also partially cancelled by a hybrid circuit contained in line interface circuits. Hybrid circuits have a variety of component arrangements but are well known to those skilled in the art of telecommunications transceiver design.
In recent years, the demands for an increase in transmission speed of full duplex communications over a two wire loop have placed greater demands on the technology associated with the transceivers providing the service. New modulations methods have been developed, such as described in U.S. Pat. No. 5,809,033 to M. Turner et al, issued Sep. 15, 1998, entitled: xe2x80x9cUse of Modified Line Encoding and Low Signal-to Noise Ratio Based Signal Processing to Extend Range of Digital Data Transmission Over Repeaterless Two-Wire Telephone Link: (hereinafter referred to as the ""033 patent) assigned to the assignee of the present application and the disclosure of which is herein incorporated.
In order to approach maximum channel capacity, the highest data rate possible on a two wire loop, it is necessary to limit the transmission degradation caused by channel impairments. Micro-interruptions, one such channel impairment, can cause performance degradation and in some cases cause loss of a transmission signal (LOS). Micro-interruptions may be caused by faulty splices, degradation of protective circuits, bad connects to equipment, and other faults. Micro-interruptions are defined as xe2x80x9ca temporary interruptionxe2x80x9d in the Draft Standard for HDSL2, T1E1.4/99-006R6, Dec. 6-10, 1999. The inventor has discovered, by computer analysis of field data that micro-interruptions generate echoes with nonlinear components. The nonlinear components, when having the magnitudes as observed from field data, cannot be cancelled by a combination of conventional echo cancelers and hybrid circuits.
Micro-interruptions may occur, for example, where a passing railroad locomotive generates vibrations in the vicinity of a manhole containing a faulty splice, either the splice itself or protection circuitry may act as a diode, and introduce excessive second order harmonics in the line. Analysis by the present inventors has revealed that a faulty splice, when subjected to ambient vibration, may temporarily change the impedance of the line, albeit only slightly (e.g., as low as three ohms). Although there is no significant change in the insertion loss of the loop, if the impedance change is very sudden (or effectively instantaneous), it can cause the transceiver""s hybrid circuit to deviate from its original response, disrupting service.
Because the full-duplex operation of the extended distance communication scheme described in the ""033 patent relies on a high level of echo cancellation, then should the impulse response of the echo path change suddenly, this high-level of echo cancellation can be lost. As a consequence, signal quality drops significantly, which, in turn, causes cyclic redundancy check (CRC) errors and finally a loss of signal (LOS).
One potential solution to the faulty splice problem would be to improve the loop environment, for example, by having an installer check and repair all of the splices near the remote terminal unit. This is obviously a tedious procedure, and it solves the problem for only a single line. A pulsing current could also be used to neutralize the bad splices. However, this would improve the loop condition only temporarily.
In accordance with the present invention, the above-described micro-interruption problem is effectively mitigated by means of a new and improved non-linear echo canceler architecture, that is able to rapidly re-adapt to the new impulse response of the effective echo path. Pursuant to a first embodiment, the invention employs both a fast, linear echo canceler, and a truncated non-linear Volterra canceler coupled to an error cancellation location in the data signal flow path downstream of the data pump""s linear equalizer. While the fast echo canceler provides superior tracking capability to combat time-varying loop impedance, the non-linear Volterra echo canceler is used to provide as much performance margin as possible, and thereby provide a very reliable link over a loop that may contain one or more micro-interruptions. The probability density function and the power spectral density of the residual noise reveal that a transceiver incorporating the first embodiment architecture does not have an asymmetrical noise distribution. As a result, the bit error rate can be predicted using Gaussian assumption analysis, and it is unnecessary to use a correction factor to adjust the margin.
Pursuant to a second embodiment, the invention employs only the nonlinear Volterra echo canceler, the output of which is coupled to an error cancellation location in the data signal flow path upstream of the data pump""s linear equalizer. In addition, the data pump""s conventional echo canceler has an increased steady-state gain for selected taps to track variations in the echo path. The advantage of using this second architecture is its reduced complexity, and the required number of taps for the nonlinear canceler is smaller. Its echo tracking performance is less than that of the first embodiment.
In a third embodiment, a high-gain echo canceler is coupled upstream of the linear equalizer, and a non-linear echo canceler is coupled downstream of the linear equalizer and between the modulo device and the decoder. The architecture of the third embodiment enjoys the benefits of the second embodiment, including similar performance, plus the additional benefits of fast tracking and low complexity.